1. Field of Invention
The present invention relates to four wheel steering systems for vehicles and more particularly front and rear wheel steering controlled by steering input from a driver operating a steering wheel. Typically, such systems provide that the rear wheels turn in the same direction as the front wheels in modes of operation associated with high vehicle speeds and in opposite directions in modes of operation associated with low vehicle speeds and intermediate degrees of turn of the rear wheels at speeds intermediate between such high and low speeds.
2. Description of Prior Art
The prior art in this area includes U.S. Patent specifications No. 3,596,730 (Cecce); 4,313,514 (Honda); 4,467,885 (Honda); 4,483,547 (Honda); 4,660,844 (Honda); 4,572,316 (Mazda); 4,646,867 (Mazda); 4,730,839 (Mazda); 4,733,878 (Mazda); 4,781,261 (Toyota); 4,770,264 (Lotus). As exemplified by these patents, innovation in the area of four wheel steering has been extremely diverse in terms of the technical solution to the problem. Surprisingly, as yet, this has not resulted in widespread adoption of such steering systems in the market-place notwithstanding that test reports indicated that vehicles incorporating four wheel steering provide several advantages including improved safety in high speed driving Two presently marketed systems are broadly covered by Honda U.S. Pat. Nos. 4,313,514 and 4,467,885 and Mazda U.S. Pat. Nos. 4,572,316 and 4,730,839 and have been described extensively in the technical literature. Information concerning the current commercial system adopted by Mazda in their MX-6 vehicle can be found in Car Australia magazine of February 1989 at page 52 et seq and a Technical Information document published by Mazda Australia Pty Limited entitled: "The Mazda Speed Sensing Computerised 4-Wheel Steering System". The present invention will be described primarily with respect to these two examples of the prior art. In the Honda system, the rear steering is entirely mechanically driven and not directly speed dependent, whereas the Mazda system is speed dependent and therefore more relevant to the present invention.
The reluctance in the market broadly adopting four wheel steering systems may reside in that they are costly, add considerable weight to the vehicle, and have serious functional shortcomings. Also, it may be that those systems enforce compromises in the design of associated front and rear suspensions which result in less desirable suspension performance. In many cases their performance depends upon electronic signals between front and rear steering gears which, in the event of a malfunction, may cause serious vehicle control problems for the driver. Where the front and rear steering gears are mechanically linked by a rotating shaft, as is the cases of Honda and Mazda, less reliance is placed on electronic signal transmission, but there is a marked deficiency in road "feel" to the driver due to high levels of friction of the respective rotating shaft arrangements.
In the Honda Prelude arrangement, a relatively conventional rack and pinion power steering gear is provided in the front steering gear having an additional rack which drives a pinion and rotatable shaft extending to the rear of the vehicle where another full mechanical steering gear is located. The rear steering gear incorporates a function generating mechanism which provides the reversing of the direction of rear steer as a function of steering wheel angle. Since each element of the mechanical linkage imposes additional friction, the overall system is characterised by having poor steering "feel" and poor self-centering action. The front to rear steering shaft requires seals, journals and universal joints at each end which not only add to the friction but, due to the great length of such a shaft, also results in a degree of compliance. To minimize the resulting loss of steering response the components are very robust and accurately fitted, and are therefore heavy and expensive.
In the Mazda MX-6 arrangement magnitude and direction of the rear-steer is made to be a function of not only front steering wheel angle but also vehicle speed. The rear steering gear incorporates a nutating axis or bobbing axis device inclinable either side of a nul position, providing the same-direction rear-steer at high speeds, opposite direction rear-steer at low speeds, and no rear-steer at a nul position corresponding to a "cross-over" speed. The axis inclination is varied according to vehicle speed by a stepper motor. By this means a single mechanism provides two separate functions of the rear steering gear, one speed dependent and one front-steer angle dependent. The rear steering gear also incorporates a hydraulic servo-system to provide power actuation to the rear wheels. In this case at least four separate power and/or signal connections are required between the front and rear steering gears.
A rotatable shaft extending between front and rear steering gears is driven by an additional rack and pinion arrangement incorporated in the front steering gear. This arrangement provides the front-steer angle input to the rear steering gear and serves as a fail-safe back-up in the event of hydraulic or electrical failure, at least up to moderate steering loads which apply during medium to high speed driving. Steering loads in parking can be several times as great, and in order to protect the nutating device from damage in a power-off situation, the excess loads are relieved by a spring loaded device which suffices to steer the car once it is in motion. In this Mazda arrangement an overload protection device is positioned on the output or highload side or the rear steer power cylinder in the form of a relatively heavy spring which also functions as a centering device in case of failure of electronic or hydraulic systems. In its function as an overload protector it acts to prevent damaging load being transmitted to the "phase control unit" in the case where high steering resistance loads are applied to the rear steer wheels such as when a rear wheel is parked too close to a curb while the front free to turn.
Also, providing the additional rack in the front steering gear increases the rack length and hence the pivot length between tie rod attachments, enforcing the use of shorter tie rods than optimal, and hence stiffer suspension spring rates.
The required level of robustness of the mechanical linkage, again therefore in this case, introduces friction to the overall steering system, and is difficult to accommodate safely under the relatively flat floor pan of modern front wheel drive vehicle.